In a typical mobile wireless communication system, received signal quality (RXQUAL) is measured by the mobile station and used for a number of purposes. Received signal quality is used by the mobile station and base station, for example, to adjust its transmit power level in order to maintain the established link. When the channel conditions are good, the received signal quality will be relatively high, and the mobile station and base station will reduce its transmit power to reduce co-channel and adjacent channel interference. Reduction of the transmit power of the mobile station also saves battery power. As channel conditions worsen, received signal quality will decline assuming no change is made in the transmit power level. This decline will be reflected in the bit error rate which is often used by proxy to estimate received signal quality. When there is a decline in the received signal quality (or an increase in the error rate), the mobile station and base station will increase its transmit power to maintain some predetermined minimum signal quality standard.
There are many schemes used to estimate the bit error rate or received signal quality. One technique is to estimate the carrier to noise (C/N) ratio and then to map the C/N ratio to a bit error ratio or signal quality band. This scheme is very well suited for analog radio systems. One disadvantage, however, is that this scheme gives very poor performance at low C/N ratios.
Another technique is known as the re-encode and compare technique. In this scheme, the output of the channel decoder is re-encoded and compared with the demodulated bits (the received bits before the channel decoder). This comparison yields an estimate of the channel bit error rate. The estimated bit error rate of the channel is then mapped to a signal quality band.
The re-encode and compare scheme yields better performance than the C/N estimation technique described above at low C/N ratios. One trade-off, however, is that at low bit error rates, the re-encode and compare scheme needs large measurement periods (i.e., a larger number of bits) to estimate the bit error rate.
The re-encode and compare scheme is not without disadvantages. The performance of the re-encode and compare scheme depends on the amount of bit errors occurring after channel decoding. If there are no bit errors after channel decoding, the re-encode and compare scheme gives an accurate estimate of the channel bit error rate. However, the performance of the re-encode and compare scheme degrades as the number of bit errors occurring after channel decoding increase. Often, the performance of the re-encode and compare scheme may not be sufficient to meet the system requirements. Another disadvantage is that the re-encode and compare scheme needs a very low code rate (high coding gain) channel coder. Since the channel coder is specified for the system, this is not always possible.
A modified version of the re-encode and compare scheme is known which attempts to overcome some of its disadvantages. The modified re-encode and compare technique is essentially the same as previously described except that the re-encoding and comparison are done only for frames that pass a CRC check following the channel decoder. That is, a CRC check is performed on the output of the channel decoder before the output is re-encoded and compared to the demodulated bits.
The modified re-encode and compare scheme provides better performance than the conventional approach. However, the modified re-encode and compare scheme can be used only in cases where CRC bits are used for error detection. Hence, it may not always be possible to implement the modified re-encode and compare scheme. Another drawback is that the modified re-encode and compare scheme skips frames that fail the CRC check. There may not be sufficient frames left over one measurement period to estimate the channel bit error rate. In such case, it may be necessary to increase the measurement period and/or the number of frames which is not always possible.
Some systems will have CRC protection for only a few important bits. For example, the transmitter bits may be divided into two classes (referred to herein as class 1 bits and class 2 bits). The number of class 1 bits, which are protected by a CRC code, is relatively small as compared to the total number of bits. Since the CRC is applied for a relatively small number of bits, the received burst after frame erasure can still have bit errors. Also, it is possible that the erased frames can have fewer bit errors than the CRC passed frames. In these cases, the modified re-encode and compare scheme may not improve performance.
Accordingly, there is need for further improvement in the re-encode and compare scheme to estimate the channel bit error rate and/or received signal quality.